The present invention relates to substrates for mirrors for EUV lithography. Moreover, the present invention relates to substrates for mirrors for EUV lithography having an essentially periodic distribution of the thermal expansion coefficient. Furthermore, the present invention relates to a method for the production of a substrate for mirrors for EUV lithography, and to a mirror for EUV lithography.
To be able to produce ever finer structures in, for example, the production of semiconductor components with lithographic methods, light of an increasingly shorter wavelength is used. When working in the extreme ultraviolet (EUV) wavelength range, at a wavelength, in particular, between about 5 nm and 20 nm, it is no longer possible to work with lens-like elements in the transmission mode, rather exposure and projection objectives composed of reflective optical elements are constructed, having reflective coatings adapted to each working wavelength.
As a substrate material, in particular for reflective optical elements for EUV lithography, so-called zero-expansion materials are used, having a thermal expansion coefficient in the near zero range at temperatures present during lithography operation and at room temperature. Prime candidates are glass-ceramic materials and quartz glass doped with titanium. Cordierite, a mineral from the silicate class of minerals, can also be used, where the thermal expansion coefficient can be influenced by adding various additives. All three material classes can be produced in such a manner that, at a temperature dependent on the actual material used, the thermal expansion coefficient, here defined as a differential for the temperature of the relative longitudinal expansion as a function of the temperature, becomes equal to zero. This temperature is also called the zero-crossing temperature. With titanium-doped quartz glass and cordierite, the zero-crossing temperature can be influenced by the content of additives, in glass-ceramic materials by recrystallization processes in strictly controlled reheating cycles.
For use as a substrate material for mirrors for EUV lithography, care has to be taken that the thermal expansion is as small as possible in the temperature range between room temperature and the operating temperature during the lithography process. To be able to fulfill the imaging precision requirements desired for EUV lithography, among other things, the tolerances for the three-dimensional profile of the optically used surfaces of the mirrors are substantially smaller than, for example, in lithography using ultraviolet radiation.